Such a method involves a first step during which the parisons are heated, within an appropriate heating unit, then a second step during which the parisons are introduced, hot, into a multiple-mold blow-molding or stretch-blow-molding unit where they are shaped into containers.
On leaving the blow-molding or stretch-blow-molding unit, the containers thus formed will be directed either toward a storage unit to await subsequent filling or directly toward a filling unit.
Let us remember that a container parison comprises a neck, intended to take the closure that seals the container that is to come and which is already at its final dimensions, extended by a body, the shaping of which will lead to the actual container proper.
The heating of the parisons is generally performed within an oven equipped with an array of tubular halogen lamps past which the parisons progress, while being rotated on themselves. More specifically, an oven contains several elementary modules, each containing several lamps, each of the lamps being controlled individually so that, ultimately, on leaving the oven, the temperature of the body of each of the parisons is above the glass transition temperature of their constituent material and a heating profile is obtained on each parison, which profile is predetermined such that the distribution of material is optimized in the container that is to be obtained.
This method of heating does have a certain number of disadvantages.
First, its energy efficiency (that is to say the ratio of the power absorbed by the parisons to the power consumed by the lamps) is extremely low, of the order of 11 to 15%. This is because of the spatial diffusion of the radiation emitted by the lamps, only a fraction of which reaches the body of the parisons. The low value displayed by this efficiency has a negative impact on production rates.
Next, the heating profile (that is to say the plot of temperatures measured along the length of the parison) cannot be obtained precisely; given the diffusion effect, the radiation from the lamps interferes with each other which means that seeking precisely to regulate the intensity of the combined radiation at a given distance from the lamps is an extremely fanciful notion.
In order to alleviate this disadvantage, there has already been the idea to make the parisons file past the lamps at the closest possible range. However, this then gives rise to an undesirable problem of overheating at the surface of the parisons, which phenomenon cannot be lessened unless an expensive ventilation system is fitted and operated.
Furthermore, there is also a significant phenomenon of thermal convection whereby the ascending air streams transfer some of the emitted radiation to the capital part of the parison. Now, the neck of this parison needs to be kept at a modest temperature so that it maintains its original dimensions.
Hence, in order to limit the incident heating of the neck by thermal convection, it has become judicious to orient the parisons neck down. As such a precaution proved to be insufficient in certain instances, it was combined with ventilation of the neck. Whatever the case, this orientation of the parisons entails, on entering the heating unit, an operation of inverting the preforms, because the preforms are generally introduced into the oven neck up, and also an operation of inverting either the preforms before they are introduced into the mold when the stretch-blow-molding step is performed neck up (which is the more common scenario), or of the containers as they leave the installation so that they can be stored or filled. These inverting operations entail installing and operating appropriate devices which make the installation more complicated and have a negative impact on cost.